[dejaentendu0]

If she takes the birth control pills are directed, every day, then there is little to worry about. The pill are about 92-99% effective, depending on if she's doing them as directed.
I never skipped the sugar pills with mine, so I don't know if that would make a difference, but I'm going to go with it won't since she's still taking the hormones - just on a daily basis without a break. However, I don't know if it's exactly healthy for her to keep doing that to miss her period. Maybe someone knows more on that though...

Most pregnancy tests can detect it pretty well at five days before. If you're still having a feeling that she might be, wait a few days until after she is supposed to have her period and maybe test then? Or if you feel worried enough, you could always have a blood test for pregnancy.
I wouldn't really worry about it though, as long as she's taking her birth control as she should and the first test was negative.